The present invention relates to an optical semiconductor device.
Recently, optical semiconductor devices have been developed. However, these optical semiconductor devices have respective problems as shown in the following description.
For example, a blue light source has been vigorously developed to become the light source for a display and the next generation DVD. A GaN-based semiconductor has particularly attracted attention since its emission wavelength can be shortened to an ultraviolet ray. However, in this material system, the electron effective mass is large so that it is hard to cause population inversion. Accordingly, this material system has disadvantages in that a threshold value is essentially high in comparison with the conventional material system. Therefore, a blue laser is difficult to obtain for practical use.
A semiconductor laser using intersubband transition within a conduction band instead of interband transition conventionally utilized has attracted attention as a technique for improving performance of the semiconductor laser. However, in this element structure, a carrier relaxation is caused by LO phonons at a very high speed so that it is hard to cause population inversion. Therefore, an intersubband transition laser is difficult to obtain for practical use.
Various kinds of optical devices for processing information have been vigorously developed as an amount of transmitted information has increased. It is particularly important to develop an optical operational element such as a bistable element. However, a bistable element having excellent characteristics has not been realized yet.
In optical communication, a direct intensity modulation by a multimode optical fiber is mainly used. However, recently, coherent optical transmission using a single mode optical fiber has been vigorously researched. In coherent optical transmission, a frequency shift keying (FSK) or a phase shift keying (PSK) is applied instead of direct intensity modulation. It is known that among these modulation methods, PSK shows a most preferable S/N ratio. However, a switching speed of a magneto-optic element, a liquid crystal or an electro-optic element used in phase control of a laser beam is generally low. Therefore, it is difficult to have a wide band width practical communication system by using phase shift keying (PSK). It is necessary to obtain linearly polarized light or circularly polarized light at a high switching speed in order to have a practical sysytem.
Linearly polarized light and circularly polarized light are also used in the fields other than optical communication. For example, a method for investigating a spin state of a substance has been proposed, in which a scanning tunneling microscope (STM) having a GaAs probe is used and the GaAs probe is irradiated with circularly polarized light for pumping conduction electrons having a spin in a specific direction. Further, semiconductor devices utilizing spin polarized electrons have been proposed. A circularly polarized laser has attracted attention as a light source for spin-pumping in such semiconductors.
As mentioned above, the importance of a circularly polarized laser and a linearly polarized laser are increasing. However, polarized light is conventionally obtained by first generating an unpolarized laser beam and then passing the laser beam through a Pockels cell or a filter. Therefore, the construction of the device is large-sized. Further, when a polarizing direction is modulated, the device becomes complicated and large-sized. Accordingly, it was difficult to make the device compact and to reduce the cost of the device.
In contrast to this, an optical semiconductor device is proposed (international publication WO95/00975), in which a spin polarized electron is injected through a magnetic layer into a semiconductor layer forming a heterojunction and circularly polarized light is oscillated by recombination of spin polarized carries. In this optical semiconductor device, the circularly polarized light is modulated by magnetization inversion to the magnetic material caused unused by an external magnetic field. However, because the response of magnetization inversion by the external magnetic field in the magnetic material is not high it is hard to modulate the circularly polarized light at high speed. BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a GaN-based semiconductor laser for obtaining laser oscillation at a low threshold value by enabling easy population inversion. Another object of the present invention is to provide an optical bistable element having excellent characteristics. A further object of the present invention is to provide a semiconductor laser having a simple construction which can be made compact and shows high speed switching characteristics of circularly polarized light applicable to optical communication by phase shift keying (PSK).
An optical semiconductor device of the present invention has a structure in which a semiconductor active layer is sandwiched by a p-type semiconductor cladding layer and an n-type semiconductor cladding layer, and a p-type contact layer is formed on the p-type semiconductor cladding layer side and an n-type contact layer is formed on the n-type semiconductor cladding layer side, wherein said semiconductor active layer is a strained quantum well layer including at least one GaN layer, and a ferromagnetic layer is formed on at least one of the contact layers. In this structure, a population inversion state of carriers is easily obtained and a GaN-based semiconductor laser having a low threshold value for laser oscillation can be realized.
Another optical semiconductor device of the present invention has an intersubband transition structure in which a semiconductor active layer is sandwiched by two semiconductor cladding layers having the same conductivity type, and contact layers are formed on either side of the two semiconductor cladding layers, wherein a ferromagnetic layer is formed on at least one of the contact layers. In this structure, a population inversion state of carriers is also easily obtained and an intersubband transition laser having a low threshold value for laser oscillation can be realized.
Another optical semiconductor device of the present invention has a structure in which a semiconductor active layer is sandwiched by two cladding layers and a resonator is formed, and also comprises means for irradiating said semiconductor active layer with circularly polarized pumping light. In this structure, an optical bistable element of high performance can be provided.
A further optical semiconductor device of the present invention has a structure in which a semiconductor active layer is sandwiched by a p-type semiconductor cladding layer and an n-type semiconductor cladding layer, and a p-type contact layer is formed on the p-type semiconductor cladding layer side and an n-type contact layer is formed on the n-type semiconductor cladding layer side, wherein two or more ferromagnetic layers are formed on the n-type contact layer, and two or more ferromagnetic layers are formed on the p-type contact layer. In this optical semiconductor device, it is preferable that two ferromagnetic layers are formed on the n-type contact layer and two ferromagnetic layers are formed on the p-type contact layer, and that magnetization directions of a pair of ferromagnetic layers vertically opposed to each other are parallel to each other, and magnetization directions of adjacent ferromagnetic layers are inverted to each other. In this structure, a semiconductor laser showing high speed circularly polarized light switching characteristics can be provided.